1. Field of the Invention
The present invention relates to an exposure apparatus for repetitively exposing a predetermined original image (circuit pattern image, and the like) onto a plurality of regions of a photosensitive layer coated on a semiconductor wafer or a glass substrate for preparing a photomask.
2. Related Background Art
In manufacture of semiconductor elements (more specifically, VLSIs and the like), a pattern on an original (e.g., a reticle or a photomask) is optically exposed onto a wafer as a photosensitive substrate. An exposure apparatus according to the present invention adopts a so-called step-and-repeat method (to be referred to as an SR method hereinafter) wherein, in order to two-dimensionally align a plurality of circuit patterns (also called chips) on the entire surface of the wafer, the wafer is repetitively exposed while it is two-dimensionally stepped with respect to the projected circuit pattern image. In projection lens performance, a minimum line width of a pattern (so-called resolution) to be projected on a wafer is very important. However, as the resolution is increased, the focal depth of the projection lens is decreased accordingly, and a line width of only several micrometers is often obtained. This means that if a deviation of the wafer surface (the photosensitive layer) to be exposed on a given stage from the focal plane of the projection lens falls outside those several micrometers, a predetermined resolution cannot be obtained in the entire area for each shot.
A wafer to be exposed inevitably has some kind of surface deformation. The wafer becomes warped by repetitive exposure, development, and wafer processing (i.e., diffusion, etching, and the like) steps, and has poor flatness. When a wafer is placed on a wafer chuck of the exposure apparatus and is drawn by vacuum suction, the wafer deformation as described above can be corrected to a certain extent, but the wafer cannot be flattened perfectly. During the operation of the exposure apparatus, a photoresist coated on the wafer is removed in the form of a powder, and when another wafer is placed on the wafer chuck, the resist powder is sandwiched between the wafer and the wafer chuck and locally impairs the flatness of the wafer surface.
The poor resolution caused by warping of the wafer surface often occurs not only with a projection type exposure apparatus, but with a proximity type X-ray stepper. In the X-ray stepper, exposure is performed while a mask and a wafer are separated by a given distance (e.g., 10 to 50 micrometers). For this reason, the irregular distance between the mask and the wafer causes the poor resolution.